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I upgraded to a new square taper crankset on a single-speed drivetrain and now the gap between my crank and the bottom bracket is bigger. That's because the cranks taper shape is different from the one I had before and I haven't changed the bottom bracket. This (along with a different shape of the crank) made the Q-factor bigger. I wanted to reduce it, so I ordered a new bottom bracket with a shorter spindle. I checked the measurement already and it's going to be all good with chainstay clearance. The question is:

How close can a square taper crank be to the bottom bracket? How small can the gap in the picture be?

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The cranks should not touch the frame or bottom bracket cup and some fraction of a millimeter is probably good for tolerance. For an example, with last generation square taper Chorus/Record the tolerance is roughly one millimeter on left side of BSA bottom bracket and on the right side the crank actually reaches into the cup. With Italian bottom bracket it is going to be even closer fit. Your crank shape doesn't allow this, so there should be a slight gap outside bottom bracket shell.

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  • In other words, can the gap I have now (~5mm) be reduced?
    – Alessandro Cosentino
    Sep 22, 2018 at 11:12
  • Yes, by roughly 4mm if the cranks do not touch chainstays.
    – ojs
    Sep 22, 2018 at 12:05
  • I accepted this answer because of the reference to an actual combined system, which was a good idea I hadn't thought about.
    – Alessandro Cosentino
    Sep 23, 2018 at 19:46
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You can reduce the gap between the crank and frame to essentially zero as long as the crank does not rub in the frame. However, that is not really how square taper cranksets are supposed to be run.

What's probably happened is you got a crankset that is designed for a shorter BB spindle. Of greater concern than the gap is that your drive side crank with the chainrings is pushed outboard and your chain-line has been messed up.

What you need to do is find the correct spindle length to give you the appropriate chain-line.

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  • What I wanted to hear :) That's on a single-speed bike and the chainline doesn't look very messed up. I am going to try reduce the spindle length from 115mm to 110mm and even futhre to 107mm.
    – Alessandro Cosentino
    Sep 22, 2018 at 11:52
  • @AlessandroCosentino Fair enough. You are taking clearance between chainring and frame into account which is important. Just be aware than having your chainring misaligned from sprocket may be noisier, less efficient and will accelerate wear on the chain, `ring and sprocket.
    – Argenti Apparatus
    Sep 22, 2018 at 11:59
  • @AlessandroCosentino Nice looking welds on that frame BTW
    – Argenti Apparatus
    Sep 22, 2018 at 11:59
  • Thanks, that's a Cinelli Mash Work frame 1st generation, I love the welds as well.
    – Alessandro Cosentino
    Sep 22, 2018 at 16:55
  • FWIW, If the old crankset was campy and new is Shimano/ JIS, that would likely be the cause of the different spacing.
    – whatsisname
    Sep 22, 2018 at 19:06

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